1. Field of the Invention
The present invention relates to an optical coupling device for coupling an optical signal propagating in an optical fiber or space to an optical waveguide.
2. Description of the Related Art
A spot size converter that performs high-efficiency optical coupling to a wire optical waveguide generally uses:
1) a method of suppressing the loss caused by the reflection or scattering of light by using a tapered structure that adiabatically compresses the mode sectional area of light; or
2) a method of combining the tapered structure and a clad for optical confinement (T. Tsuchizawa et al., IEEE Journal of Selected Topics in Quantum Electronics, Vol. 11, pp. 232-239, 2005)
The first method has a wide application range and is most often used. However, it is difficult to apply this method to optical coupling to an Si-wire waveguide for which the mode sectional area of light must be reduced by about two orders of magnitude from several μm×several μm to a few hundred nm×a few hundred nm. The second method using the advantage of silicon for which the advanced micropatterning techniques can be used is used for optical coupling to an Si-wire waveguide requiring the large reduction in mode sectional area as described above.
In an optical coupling device using the second method, a buried insulating film (SiO2) of an SOI substrate is used as a lower cladding layer, a wire optical waveguide made of Si is formed on the lower cladding layer, and an upper cladding layer a few μm square made of a polymer or SiON is buried in this wire optical waveguide. The wire optical waveguide has a height of about 200 to 300 nm and a width of about 300 to 500 nm. A tapered structure serving as an input/output portion has a distal end width of 100 nm or less and a length of about 200 to 300 μm. Light having entered the upper cladding layer is gradually coupled with the silicon-wire waveguide in the tapered structure. Accordingly, 90% or more of light can be coupled with the wire waveguide when the taper length is about a few hundred μm.
Unfortunately, an optical coupling device of this kind has the following problem. That is, a three-dimensional structure several μm or more square is formed as the upper cladding layer by performing a process such as vapor deposition or annealing by using, e.g., a polymer such as polyimide resin or SiON. This makes it difficult to perform integration by matching the process with that of another optical device such as a light-receiving or light-emitting element and its electrode. This makes large-scale integration and mass production difficult.